Energy & Fuels 2003, 17, 457-461
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Effect of Initiative Additives on Hydro-Thermal Cracking of Heavy Oils and Model Compound Jie Chang,† Kaoru Fujimoto,‡ and Noritatsu Tsubaki*,§ Guangzhou Institute of Energy Conversion, No. 81 Xianliezhong Rd., Guangzhou, 510070, P. R. China, School of Environmental Engineering, The University of Kitakyushu, Hibikino, Wakamatsu, Kitakyushu, Japan 808-0135 Department of Material System and Life Science, School of Engineering, Toyama University, Gofuku 3190, Toyama, Japan, 930-8555 Received September 4, 2002
The hydro-thermal cracking of heavy oils, such as Canadian oil sand bitumen and Arabian heavy vacuum residue, as well as their model compound, was performed over sulfided Ni/Al2O3 and NiMo/Al2O3 catalysts under 663-703 K and 5.0-8.0 MPa of hydrogen pressure in a batch autoclave reactor. According to the reaction mechanism of hydro-thermal cracking, some free radical initiators, such as di-tert-butyl-peroxide (DTBP), sulfur, etc., were added into the feed to generate free radicals at lower temperature, and some initiators did obviously show a promotional effect on the conversion of hydrocarbons. The reaction mechanisms of hydro-thermal cracking as well as the enhancing effect of initiators were studied by a probe reaction with 1-phenyldodecane as a model compound under the conditions of hydro-thermal cracking. The hydro-thermal cracking of hydrocarbons proceeded via a free-radical mechanism and hydrogenating quench. The initiators might easily generate free radicals under the reaction temperature, these radicals might abstract H from hydrocarbon molecules and reasonably initiated the chain reactions, therefore, promoted the conversion of hydrocarbons even at lower reaction temperature. The reaction temperature could be lowered by the addition of a free radical initiator, while keeping the same conversion level.
Introduction Due to the limitations in supplies of petroleum crude, current industrial and research attention have increasingly focused on the conversion of heavy oils for the production of gasoline, lubricants, and chemicals. Recent developments in residue hydroprocessing reflect the worldwide trends in petroleum product demands and crude oil quality. With the continuous increase of the crude oil price, one of the challenges of 21st-century petroleum refinery is to convert more heavy portion of crude oil into valuable transportation fuels. Hydrothermal cracking is one of the important hydroprocessing processes, which was developed to provide flexibility in refining to make gasoline and light oils from less valuable petroleum stocks, such as oil sand bitumen, residua, and vacuum gas oil.1 Hydro-thermal cracking is different from the traditional hydrocracking. On one hand, many commercial hydrocracking catalysts consist of transition or noble metals distributed in/on zeolite supports, with the metal providing the hydrogenation sites and the highly acidic support providing cracking activity via a carbonium ion mechanism. Amorphous silica-alumina supports can * Corresponding author. Tel/Fax: (81)-76-445-6846. E-mail: tsubaki@ eng.toyama-u.ac.jp. † Guangzhou Institute of Energy Conversion. ‡ The University of Kitakyushu. § Toyama University. (1) Chang, J.; Tsubaki, N.; Fujimoto, K. J. Pet. Inst. Jpn. 2000, 43, 357.
also be used to provide the cracking activity, but its cracking activity is lower than that of zeolite because of its lower density of acidic sites. Catalysts for the heavier feedstock often contain metal sulfides instead of metals, and amorphous supports instead of or in addition to zeolites.2 On the other hand, hydro-thermal cracking is the combination of thermal cracking and catalytic hydrogenation. The cracking of heavy oil is mainly from thermal reaction via a free-radical mechanism. The hydrogenation sites are provided by alumina-, silica-, or activated carbon-supported transition metal sulfides. The acidity of this kind of support is too weak to supply cracking activity like a hydrocracking catalyst. The initial step in the cracking sequence is the formation of a radical from the original paraffin. The relative strength of bonds of hydrogen attached to a particular carbon is primary > secondary > tertiary. The mechanism of hydrocracking of residues3,4 and the chemical changes in the different product fractions under different catalysts5,6 have been studied intensely. Many researchers have extensively discussed the role of a catalyst in hydrocracking of heavy oils and concluded that the main role of a catalyst in hydrocracking (2) Lapinas, A. T.; Klein, M. T.; Gates, B. C.; Macris, A.; Lyons, J. E. Ind. Eng. Chem. Res. 1987, 26, 1027. (3) King, P.; Morton, F.; Sagarra, A. Modern Petroleum Technology; Hobson, G. D., Pohl, W., Eds.; J. Wiley: New York, 1973; p 186. (4) Beaton, W. I.; Bertolacini, R. J. Catal. Rev. 1991, 33, 281. (5) Calafat, A.; Laine, J.; Lopez-Agudo, A.; Palacios, J. M. J. Catal. 1996, 162, 20. (6) Medici, L.; Prins, R. J. Catal. 1996, 163, 28.
10.1021/ef020190u CCC: $25.00 © 2003 American Chemical Society Published on Web 02/15/2003
458 Energy & Fuels, Vol. 17, No. 2, 2003
Chang et al.
Table 1. Properties of Feedstock API gravity CCR C H S N Ni V saturates aromatics resins asphaltenes residue (798 K+)
[wt %] [wt %] [wt %] [wt %] [wt %] [wt ppm] [wt ppm] [wt %] [wt %] [wt %] [wt %] [wt %]
bitumen
AVR
6.0 14.8 82.89 10.14 4.90 0.45 75 192 10.9 61.5 18.1 9.5 65.2
5.9 22.4 84.80 10.20 4.92 0.43 53 180 8.3 35.6 45.4 10.7 95.0
is to supply hydrogen to large oil molecules, which otherwise become deficient in hydrogen.7-9 The development, deactivation, and regeneration of catalysts also have already received more attention in the literature.10 However, the study on the initiation method for hydrothermal cracking of heavy oil has been seldom mentioned. The conversion of residual oil mainly depends on thermal cracking, especially under higher reaction temperatures during hydro-thermal cracking. Higher reaction temperature can simply increase the conversion, but the selectivities to gaseous hydrocarbon and coke will be enhanced simultaneously even in the presence of a catalyst. The addition of initiators may lower the reaction temperature, therefore increase the selectivity to middle distillate, while keeping the conversion level. Wei et al.11 introduced the effect of sulfur on the hydrocracking of di(1-naphthyl)methane (DNM) that the addition of sulfur resulted in a marked increase in DNM conversion at 703 K. Al-Amrousi12 reported the promotional effect of phenol as an initiator on the liquefaction of polypropylene. The present authors have published preliminary results about the promotional effect of initiator (DTBP) on heavy oil hydro-thermal cracking as a communication.13 This paper will report the effects of various initiative additives on hydrothermal cracking of heavy oils, their model compounds, and its mechanism. Experimental Section Feedstock. Canadian Athabasca oil sand bitumen (bitumen) and Arabian heavy vacuum residue (AVR) were used as feedstock. Their properties are listed in Table 1. At ambient temperature, bitumen and AVR are in the semisolid state. Both of them have lower API gravity, about 6. The metal content (Ni + V) is 267 wt ppm in bitumen, and 233 wt ppm in AVR. The obvious difference between bitumen and AVR is the SARA (Saturates, Aromatics, Resins, and Asphaltenes) composition. Bitumen has more aromatics (61.5%) than AVR (35.6%) and less resins (18.1%) than AVR (45.4%). Residue content in bitumen is 65.2%; however, it is 95.0% in AVR. Hydro-thermal cracking supplies a useful method to obtain high quality middle distillates from bitumen and AVR. The addition of initiative additives gives an approach to enhance the conversion of resids. The initiative additives used were ditert-butyl-peroxide (DTBP, (CH3)3COOC(CH3)3), oxygen, and sulfur. (7) Miki, Y.; Yamada, S.; Oba, M.; Sugimoto, Y. J. Catal. 1983, 83, 371. (8) Savage, P. E.; Klein, M. T.; Kukes, S. G. Energy Fuels 1988, 2, 619. (9) Gray, M. R.; Khorasheh, F. Energy Fuels 1992, 6, 478. (10) Chang, J.; Liu, J.; Li, D. Catal. Today 1998, 43, 233. (11) Wei, X. Y.; Ogata, E.; Zong, Z.; Niki, E. Fuel 1993, 72, 1547. (12) Al-Amrousi, F. A. Fuel 1997, 76, 1451. (13) Chang, J.; Fan, L.; Fujimoto, K. Energy Fuels 1999, 13, 1107.
To study the effect of initiative additives more clearly, alkylbenzene was used as a model compound in hydro-thermal cracking. When alkylbenzene was thermally cracked or cracked on solid acid catalyst, the selectivities of benzene and toluene were so different that it could be used as a model compound to distinguish a reaction via free radical mechanism or carbonium mechanism.14 Savage and Klein studied the thermal cracking of 1-phenyl dodecane (PhDD) and suggested that it proceeded through a free radical mechanism.15 The reaction of 1-PhDD was used as a probe reaction to study the mechanism of initiative additives. The liquid products of hydrothermal cracking of a model compound were determined by GC-MS (Shimadzu GCMS 1600) and analyzed by gas chromatography (Shimadzu GC-14A) with an NB-1 capillary column. Catalyst Preparation. Catalysts were prepared by the impregnation method using the appropriate aqueous solution of active metal salt via the incipient-wetness method. The bimetallic catalysts were prepared by the co-impregnation method. The support was γ-Al2O3 (Nikki Chem. Co., Ltd.). The supported active metals were nickel and molybdenum using Ni(NO3)2‚6H2O and (NH4)6Mo7O24‚4H2O precursors, respectively. After impregnation, the catalysts were degassed in a vacuum, and then dried at 393 K for 12 h in air, followed by calcination at 773 K for 4 h in air. Before use, the catalyst was reduced in a 120 cm3/min hydrogen flow at 723 K for 4 h and then sulfided in a 20% H2S/H2 mixture at the same condition. Experimental Procedure. The reactions of bitumen hydrothermal cracking were carried out in a 75 cm3 autoclave reactor. The reactor was loaded with feedstock and catalyst, pressurized with hydrogen at room temperature, then heated and agitated at 70 rpm to ensure efficient contact between gas, liquid, and solid. The reactor took about 10 min to rise to the reaction temperature. The reaction time was calculated from the time when the desired reaction temperature was reached. After the reaction, the reactor was cooled quickly to room temperature. The gaseous product was released to a collecting bag, and the liquid product was separated from the solid. The reaction conditions were as follows: temperature, 683713 K; initial pressure, 5.0 MPa; reaction time, 0-120 min; weight ratio of catalyst/oil, 0.2. Product Analysis. H2 was analyzed by gas chromatography (Shimadzu GC-3B) with TCD. Light hydrocarbons were measured by gas chromatography (Shimadzu GC-8A FID) with an active alumina column. H2S was analyzed by a hydrogen sulfide detector tube. The liquid oil products were analyzed by a distillation-type gas chromatograph (Shimadzu GC-14A) with a silicone OV-1 glass column as 5 components: naphtha (IBP-443 K), kerosene (443-503 K), gas oil (503-616 K), vacuum gas oil (616-798 K), and residue (>798 K). The amount of coke was determined as follows: after separation from oil by filtration, the solid was subject to Soxhlet extraction by toluene for 3 h to remove oil absorbed in the solid, then dried at 393 K in a vacuum (
